NASHVILLE (Reuters) - A Tennessee appeals court ruled on Thursday that a state requirement that voters have photo identification is constitutional but that the Memphis library cards at the heart of a challenge to the law were acceptable at polling places.

The Tennessee Secretary of State plans to appeal the ruling to the state Supreme Court, a move that will freeze the appeal court ruling until the high court takes action, according to Blake Fontenay, a spokesman for the Secretary of State.

"This is like changing the rules at halftime of a football game," Fontenay said, explaining the quick reaction to Thursday's ruling, which came as early voting in the November 6 general election was already underway.

"I wouldn't lose sight of the part of the ruling that we are very pleased with that upholds the constitutionality of the law," he added.